Optical coherence tomography (OCT) is an optical imaging technology for imaging non-invasively biological tissues. The basis of the technique is low-coherence interference between a reference beam and light reflected from structural surfaces such as tissues. Depending on the scanning or imaging configuration, various dimensions can be probed such as a line (called an A-scan), transverse scanning can produce a 2D surface known as a B-scan, and with multiple adjacent surfaces a 3D volume can be obtained.
In frequency-domain OCT (FD-OCT), the optical path length difference between the sample and reference arm is not mechanically scanned as it is in time-domain OCT (TD-OCT). A full A-scan can be obtained in parallel for all points along the sample axial line within a short time, typically determined by the wavelength sweep rate of a swept source in swept-source OCT (SS-OCT) or the line scan rate of the line scan camera in spectral-domain OCT (SD-OCT).
The spectral interferogram acquired through FD-OCT encodes the longitudinal sample structure at the focal spot. To recover the sample structure, the interference pattern can be inverse Fourier transformed. This transform yields three components: a DC component, a cross-correlation component, and an auto-correlation component. The DC terms are often the largest component and are pathlength-independent. The cross-correlation terms contain the information of interest—the sample reflectivity profile. The auto-correlation terms represent interference between the different reflectors within the sample. Elements of all three of these components can lead to artifacts that can cause problems in data interpretation or processing.
Imaging of the anterior segment of an eye of a patient using OCT systems presents different problems than encountered with the more commonly performed OCT imaging of the retina. Structures in the anterior segment include the cornea, the iris, the crystalline lens, and other anatomical areas as well. Problems encountered with OCT anterior segment imaging include distortions of the acquired image from a true geometric image, the restricted nature of the depth and breadth of imaging, and the difficulty in obtaining good real-time positional control during image acquisition.
Dewarping
Dewarping is the restoration of the true shape of the cornea and environs due to physical phenomena: the effects of beam geometry distortions and distortions due to the bending of light by various refractive surfaces. These problems can be eliminated by dewarping algorithms (Westphal et al. 2002; Ortiz et al. 2009, 2010). Accurate dewarping is required for an accurate analysis of anterior segment OCT image data to characterize pathologies (such as keratoconus) or metrics of pathologies (geometric metrics or measurements of structures contained therein).
Beam geometry distortions arise due to the non-telecentric scanning of the OCT beam used to obtain a 2D image (a B-scan) and also due to the optical path length, which is related to scan sag, changing the curvature of the cornea. The beam itself encounters different angles as it scans across, as the cornea is highly curved. This results in a distorted image and thus any measurements made of structures in an undewarped image, can result in erroneous values. Other distortions are caused by light rays encountering interfaces between media of different indices of refraction. For instance, rays pass from the air (refractive index of 1.000) into the cornea (refractive index of about 1.372), and from the cornea into the aqueous humor (refractive index of about 1.332). Dewarping requires knowledge of the refractive indexes of the cornea and the aqueous humor, as well as the locations of the anterior and posterior corneal surfaces. Dewarping, using only one of the two corneal surfaces, can yield useful information. However, a critical element in the approach to proper dewarping is the ability to segment correctly one (first surface) or more surfaces used in the dewarping procedure.
Upon dewarping, more accurate metrology of the structures found downstream of the corneal surface or surfaces can be performed. For instance, anterior chamber depth and angle can be measured by using the iris and lens segmentation to guide semi- or fully-automatic measurements based on user identification of at least one reference mark in the image: such as the scleral spur but other anatomical items include Schwalbe's line, trabecular meshwork, Schlemm's canal, or a corneal or iris surface or surfaces. The posterior corneal surface and iris segmentations can also be used for semi- or fully-automatic angle measurements (e.g., the iridocorneal angle). Other useful measurements can be geometric metrics of any surface, distance, volume, or interface, applicable to the diagnosis of pathological conditions. These values can be used in planning surgical procedures such as keratectomy, trabeculectomy, and detection of pathologies such as keratoconus.
Reducing the Interference of the Complex Conjugate Images on Real Images
A major artifact in FD-OCT imaging is that of the complex conjugate, which arises from the earlier discussed cross-correlation terms due to the inverse Fourier transform used. The complex conjugate artifact is a mirror image of the true image flipped around the zero-delay—the virtual position where the optical path length in the sample arm equals that of the reference arm. In some cases, the mirror image is distorted due to dispersion mismatch in the two arms of the interferometer, causing it to appear blurred. The complex conjugate artifact can lead to ambiguity in image interpretation as well as erroneous analysis of the OCT data, particularly in the anterior segment. FIG. 1 presents two images of the anterior segment of an eye. FIG. 1(a) is one in which the overlap of real images of iris and corneal structures (real cornea image 102 and real iris image 103) with complex conjugate images of iris and corneal structures (mirror iris image 101 and mirror cornea image 104) hampers adequate analysis of the image. FIG. 1(b) is an image that will allow more straightforward identification of features of interest as the various imaged structures of real and complex conjugate are more cleanly separated than those depicted in FIG. 1(a).
In many OCT imaging situations, the relative positions of sample and reference arms (the ‘delay’ position) can be adjusted so that the sample is located entirely in positive or negative space (terms relative to the position of the zero-delay). In this case the complex conjugate part of the image is contained primarily in half of the axial range of the resulting data, and only the real part of the image is contained within the reported axial field-of-view. When imaging the anterior segment of the eye, visualization of multiple structures that extend throughout the entire axial field-of-view of the image is desirable, so limiting the reported field-of-view to half of the acquired data is not an optimal solution.
Several OCT imaging techniques capable of removing or minimizing the complex conjugate artifact have been demonstrated. (In the present application, the terminologies of mirror image and complex-conjugate image are considered to be equivalent.) (See, e.g. Wojtkowski et al. 2002; Yasuno et al. 2006; Wang 2007; Baumann et al. 2007, Zhang et al. 2005, Hofer et al. 2009).
Hardware-based approaches, unfortunately, add cost and complexity to the system, and several approaches have technical limitations. Other techniques rely on specialized algorithms to reduce or remove the complex conjugate artifact (see, e.g., U.S. Pat. No. 8,414,564). The majority of these methods require multiple frame acquisitions and heavy post-processing, which obviates real-time display, visualization, and correction.
Tracking Using Anatomical References in the Anterior Segment
Two critical requirements for successful dewarping of anterior chamber OCT image data are good placement of the corneal surfaces within the available OCT image window to maximize surface coverage, and little or no overlap of the real images with those of the complex conjugate images to permit clean segmentation of the desired surfaces. Both of these require stable control over positioning of the eye of the patient relative to the instrument, and this can be accomplished by tracking.
Due to patient motion, both lateral and longitudinal (i.e., along the scan axis) tracking of eye motion during examination and providing compensation thereto is preferred. Should the patient move longitudinally, as discussed above, then the complex conjugate and real images could overlap, causing segmentation problems, hence deficiencies in the information provided to the dewarping process. Moreover, tracking allows more accurate image registration, accurate placement of anterior segment of structures, and mosaicking of images. Anatomical reference marks for tracking are well-known for the retina, and include at least, the optic nerve head (ONH), the macula, and blood vessels (see, e.g., U.S. Pat. No. 7,884,945). However, these reference marks are not available in OCT anterior segment imaging.
Most OCT systems are optimized for retinal scanning. Anterior segment scans are possible, nevertheless, by the insertion of additional lenses or perhaps by a different optical train in the system. The depth of imaging in the anterior segment poses a severe problem, as the distance between the anterior surfaces of cornea and crystalline lens is 3.5 mm. To reach the posterior surface of the crystalline lens adds an additional 6 mm. Thus simultaneous imaging of a large portion of the anterior segment is currently not possible. Techniques need to be developed to overcome this imaging limitation.